Conventionally, the focus of a motion picture camera lens is set manually to a distance where the plane of focus of the lens closely coincides with the location of a photographic subject. The camera lenses have focus distance calibrations that correspond to the distance from the image plane of the camera to the focus plane. Motion picture cameras may also have an electronic mechanism for delivering the focus distance setting through an electrical interface. The focus puller (e.g., a technician responsible for setting the focus of the camera lens) can adjust the camera lens to the subject distance by estimating the distance to a photographic subject and setting the focus distance of the lens to match the estimated distance, either directly or using electro-mechanical controls.
To estimate the focus setting for a static camera, the focus puller conventionally uses distance cues, such as reference marks made on the ground during rehearsal, or other objects whose known position relative to the camera can serve as distance references. Provided that the focus setting corresponds to the subject being within the depth of field of the lens (this depth being a specified range of distances in front of and/or behind the focus distance setting), the subject will appear acceptably sharp.
In many situations, the focus puller cannot achieve acceptably sharp focus using these conventional methods. For example, when the camera is moving during the process of shooting a scene, it is often not possible to use pre-set focus reference marks, as the path taken by the camera may not be predictable. In other situations, the depth of field of the lens is so shallow that the focus setting cannot be reliably estimated by the focus puller, even when reference marks are available.
In the situations described in the previous paragraph, the focus puller may use the image captured by the camera as displayed by a monitor to adjust the focus. However, the monitor can only show the degree to which the image appears to be in focus. If the image or subject is out of focus, the monitor cannot show the direction or the magnitude of any necessary focus setting change that will bring the subject coincident with the plane of focus, or within the depth of field of the lens.
There have been a number of challenges to focusing a camera (or image captured by the camera) using a video monitor. For example, once unclear or “buzzed” focus is observed, it is often too late to maintain or bring back focus, especially when subjects in the field of view of the camera are in motion. Sometimes it can be difficult to tell whether one must pull forward or backward to correct the focus. Peaking, or making the edges of objects within the image more visible (e.g., by increasing the contrast or color of the edges, or making the edges shimmer) does not provide much critical focusing information. Also, on many episodic TV series, there isn't sufficient room on set for the focus pullers, and they often work outside the set. On multiple camera shoots, there will be one monitor and one wireless focus unit for each camera and each assistant, thereby making demands for space for focus pullers even higher.
U.S. Pat. No. 8,982,272 (“US '272”) describes a focus assistance system that uses graphics overlaid on the images in a video monitor to show to the focus puller both the direction and the amount that the focus setting of the camera lens must be changed to bring a subject in one or more of a plurality of detection zones into sharp focus. In addition, US '272 describes an autofocus function in which the user selects one or more detection zones, and the closest distance from a subject in the selected zone(s) to the camera image plane is used by the system to automatically focus the camera on the subject. In US '272, a distance measuring device (DMD) is mounted a distance from the entrance pupil of the camera shooting lens. As a result of this separation, the alignment of the overlay graphics with the corresponding areas of the video image must be shifted in both elevation and azimuth, with the magnitude of the shifts varying with the distance measured in each detection zone.
US '272 discloses alignment of the overlay graphics according to the focus distance set by the user and the separation between the DMD and the camera lens entrance pupil. Specifically, the video overlay and processing unit uses the known focus distance of the camera as measured from the image plane of the camera to shift the position of the overlay graphics in both the horizontal and vertical directions. Shifting the overlay graphics according to known focus setting(s) may have one or more disadvantages. First, the shift is generally valid only for subjects at the known focus setting(s), and the setting(s) are generally not known a priori. Furthermore, the camera image may show a plurality of subjects at different distances from the camera image plane, and a shift that is valid for one subject of interest may not be valid for another subject of interest. US '272 also discloses scaling the overlay graphics according to the angle of view of the camera lens, so that the detection zones are aligned with the image even as the camera field of view changes.
To react sufficiently quickly to the movement of subjects in the field of view, the focus puller must pay close attention to the camera images displayed on the video monitor. However, when the overlay graphics has a one-dimensional array of detection zones and the DMD is offset horizontally from the camera sensor or lens, the graphics (and more specifically, the position and/or size of one or more detection zones) can change as the zoom setting or focal distance changes. Thus, the focus puller or other viewer of the images on the video monitor may become distracted by shifts in the position of the overlay graphics relative to the images on the monitor, and their ability to choreograph the focus changes with changing subject distances may be adversely affected.
Additionally, the focus puller or other user may lose focus distance information resulting from parallax. When the DMD is offset vertically from the camera sensor or lens, parallax can result in difficulty obtaining accurate distance measurements in each of the detection zones when a subject or object is relatively close to the camera lens. This is believed to be due to the distance of the DMD from the entrance pupil becoming a significant fraction of the distance of the subject or object from the camera. For example, in the case where the DMD employs a one-dimensional detector array arranged in a horizontal row configuration, and the overlay graphics are offset according to the vertical parallax, the overlay graphics may be shifted beyond the bounds of the monitor image, resulting in the DMD not being able to provide distance information from an area of potential interest to the focus puller. In the case of horizontal parallax, the graphics will also shift horizontally, potentially causing the loss of distance information.
In camera focusing systems in which the angular size of the detection zones remains constant, the size of each detection zone (and in some cases, the number of displayed zones) changes as the focal length and the field of view of the camera lens changes. In particular, when the camera captures a large field of view, the number of detection zones may increase such that the graphics overlay depicting the detection zones obscures the camera image. In such systems, the maximum number of zones that can appear on the monitor may be the same as the number of detector elements (e.g., photodetectors or photodiodes) in the DMD.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.